Apparatus for inductor current control in electric arc welding

ABSTRACT

A D.C. arc welding current contains an inductor connected in parallel with a shunt branch containing a half-wave rectifier and a resistor. The rectifier is poled so as to block the current produced in said shunt branch by the voltage induced in the inductor on decay of the magnetic flux in the inductor. Timedelayed switching means responsive to a flow of welding current are provided for reducing the conductivity of the shunt branch from a first level to a second level (which may be zero).

Umted States Patent 1191 1111 3,711,058 Weman 1451 Jan. 16, 1973 [54]APPARATUS FOR INDUCTQR [56] References Cited CURRENT CONTROL IN ELECTRICUNITED STATES PATENTS ARC WELDING 3,078,362 2/l963 Steiner! ..2l9/l35[75] Inventor- Bert" weman' Lama Swede 3,291,960 l2/l966 Folkhard @1211...219 131 [73] Assignee: Elektriska Svetsningsaktieholaget,

Gothenburg Sweden Primary ExaminerJ. V. Truhe Assistant ExaminerGeorgeA. Montanye [22] Flled: 5! 1971 Att0rneyCameron, Kerkam & Sutton A DC.arc welding current contains an inductor con- [30] Forelgn Appl'cammpnor'ty Data nected in parallel with a shunt branch containing a Jan.14, 1970 Sweden ..386/70 half-Wave rectifier and resist The rectifier ispoled so as to block the current produced in said shunt 521 u.s.c1...219/131 WR 219/137 branch by the Wltage in the indum 51 1m.c1. .lizsk9/10 decay Of the magretic flux induct Time- [58] Field of Search219/131 137 5, 307/100 delayed switching means responsive to a flow ofwelding current are provided for reducing the conductivity of the shuntbranch from a first level to a second level (which may be zero).

5 Claims, 6 Drawing Figures PATENTEUJAH 16 1913 3.71 1 058 SHEET 1 [IF 2PATENTEUJAHISIQB 3 711 058 SHEET 2 0F 2 APPARATUS FOR INDUCTOR CURRENTCONTROL IN ELECTRIC ARC WELDING This invention relates to apparatus forelectric arc welding with a consumable electrode advanced at a constantrate and energized by a DC. source of welding power through a circuitincluding an inductor. More particularly, the invention is concernedwith apparatus of this type in which a winding of the inductor isconnected to a shunt branch containing a resistor in series with ahalf-wave rectifier poled to block a flow of current through said branchcaused by the voltage induced in said winding on decay of the magneticflux in said inductor. Said shunt branch, the purpose of which is tocontrol the rate of rise of the welding current, may be connected acrossthe ends of the main winding of the inductor, as shown in FIG. 8 of theUS. Pat. No. 3,291,960, Folkhard et al., or connected to an auxiliarywinding of the inductor, as shown in FIG. 2 of the U.S. Pat. No.3,078,362, E.F. Steinert.

Apparatus of the kind above referred to are known to be particularlyuseful for short arc welding (also referred to as welding withshort-circuiting metal transfer) with a source of welding power having asubstantially flat characteristic, or a characteristic exhibiting aslope not exceeding 8 volts per 100 amps. Some difficulties have,however, been caused by the fact that the conditions for the ignition ofthe are are less favorable during the starting period, in which theelectrode meets a comparatively cold workpiece, than during thesubsequent welding period proper, during which the electrode tipcooperates with a pool of liquid metal. The resulting problems wereparticularly serious in welding operations requiring an absolutelyreliable starting of the welding operation, for instance in are spotwelding. To exclude start failures, the choice of the welding circuitparameters had to deviate considerably from the one required to providethe best conditions during the welding period proper.

The invention has for its principal object to provide apparatus of thekind referred to in which the above difficulties are avoided by theprovision of means by which the apparatus is caused to provide one setof parameters during the starting period and a different set ofparameters during the welding period proper.

According to one feature of the invention, switching means are providedfor reducing the conductivity of the shunt branch in respect of currentshaving the direction admitted by the half-wave rectifier connected insaid branch from a first level to a substantially lower level, anddelayed means responsive to a flow of welding current for operating saidswitching means.

Other objects, features and advantages will appear from the followingmore detailed description of illustrative embodiments of the invention,which will now be given in conjunction with the accompanying drawings.

In the drawings,

FIG. 1 schematically represents one embodiment of the invention,

FIG. 2 is a current diagram illustrating the starting process obtainedwith apparatus according to FIG. 1,

FIGS. 3, 4 and 5 schematically represent, respectively, three otherembodiments of the invention, and

FIG. 6 is a current diagram illustrating the starting process obtainedwith apparatus according to FIG. 5.

In the apparatus according to FIG. 1, the terminals 2, 3 of a DC. sourceof welding power 1 are connected to a welding circuit comprising theinductor 4, the winding of the welding current relay 5, the contact tube6, the fusible electrode 7, the work 8 and the return cable 13. Thesource of power 1 has a flat characteristic and may for instance be arotary generator or a rectifier supplied by a three-phase transformer.The schematically indicated wire advancing mechanism 9 serves in a knownmanner to advance the electrode 7 at a constant rate. A branchcontaining a half-wave rectifier 10 in series with the back contact 50of the welding current relay 5 and the resistor 11 is connected inparallel with the inductor 4. Moreover, a by-pass resistor 12 isconnected across the series connection of the rectifier 10 and thecontact 5a.

The welding current relay 5, the inductance of which is small comparedto the inductance of the inductor 4, responds with a certain delay toenergization as well as to deenergization. The apparatus is designed forshort arc welding, the voltage of the source 1 being too low to maintaina stationary state in which the arc burns without interruption by shortcircuits. During the welding period (contact 5a open) the inductor 4 hasthe known function of controlling the short circuit frequency and therate of increase of the current in such a way as to provide a quiet andregular welding process without spatter.

In the following description of the operation of the apparatus noaccount is at first taken of the by-pass resistor 12, which may actuallybe dispensed with. At the start of the welding process, the back contact5a is closed, as there flows no welding current and the welding currentrelay 5 is not energized. At the moment t (FIG. 2) in which theelectrode tip makes contact with the workpiece, a current i flows fromthe positive terminal 2 through the rectifier 10, the contact 5a, theresistor 11, the winding of the relay 5, the contact tube 6, theelectrode 7, the workpiece 8 and the return cable 13 to the negativeterminal 3. The relay 5 is energized but owing to the delay of the relaythe contact 5a remains closed during a certain period. The current ithrough the inductor 4 increases gradually and asymptotically approachesthe static short-circuit current corresponding to the steady state. Atthe same time, the current i through the rectifier 10 decreases andasymptotically approaches zero. The current through the electrode 7 isthe sum of the currents 1' and i At the time t said current has grownsufficiently to cause fusion of the electrode, so that the short circuitis terminated and an arc is ignited. The current i,, through theinductor then tends to decrease, resulting in a change of direction ofthe inductive voltage appearing across the inductor 4. As the rectifier10 is non-conducting in this direction, the branch l01l will now becurrentless. Consequently, the welding current is reduced suddenly atthe time I, by the value i (the value of the current i,, at the time1,), and subsequently decreases gradually as the magnetic energy storedin the inductor is consumed. As the are under the conditions givencannot burn permanently, another short circuit followed by an ignitionperiod will occur after a certain time. As the electrode tip and thecontact spot on the workpiece are now hot, the ignition this time islikely to take place more easily than the first time. Consequently, theshort circuit is likely to be terminated before the current has reachedthe peak value attained in the first short circuit.

The delay of the operation of the relay 5 must be so adjusted that thecontact 5a remains closed at least during all of the short circuitperiod from t to 1,. As a rule, the arc period following the firstignition cannot be relied on to establish such conditions that thesubsequent ignitions occur with the same ease as during the normal shortarc welding process. Therefore, the delay of the operation of the relayis preferably adjusted in such a way that the contact 5a remains closedduring at least the first two short circuits. The time required toattain the normal, or steady-state, welding conditions is depending onseveral factors, such as the material of the electrode and theworkpiece, the electrode diameter and the rate of advancement.Therefore, the choice of the delay of the operation of the relay willalso be depending on said factors. In welding thin sheet with steelelectrodes of the diameters 0.6 1.2 mm, delay times of the magnitude 0.30.5 seconds have been found to be suitable.

The improved reliability of the ignition obtained with v the apparatusaccording to the invention can be ascribed to two different factors. Thefirst factor is that the current at the initiation of the short circuitimmediately assumes a comparatively high value i,,, so that there is animmediate and strong discharge of energy at the point of contact betweenthe electrode and the workpiece resulting in incipient fusion of theelectrode tip. This removes or reduces a danger caused by too lowinitialcurrents, to wit, that the electrode may come into close contact withthe workpiece and be effectively cooled by the workpiece before theelectrode tip has started to melt. In that case, the current will riseto a very high value before the short circuit is terminated by theelectrode end blowing like a fuse. The second factor is that themagnetic energy stored in the inductor at the end of the short circuitperiod is substantially lower than the one which would result from allof the short circuit current passing through the inductor. This reducesor eliminates the risk that the arc current at the beginning of the arcperiod under the action of the stored magnetic energy produces a veryrapid fusion and a resulting increase of the arc length to undesirablyhigh values, resulting in the extinction of the arc before the nextshort circuit, as the decreasing current will no longer be able tosustain an arc at the existing arc length.

The resistor 11 may be adjustable in order to allow an adaptation of theinitial current i to different electrode diameters and different ratesof advancement.

The resistor 12, which also may be adjustable, forms, when the contact ais open, in series with the resistor 11 a resistance branch connected inparallel to the inductor 12, the purpose of said resistance branch beingthe known one of controlling the shape of the current curve during thewelding period.

The conductors 14, 15 may be connected to the terminals of an auxiliaryor secondary winding of the inductor 4 instead of, as shown, to theterminals of the single winding of the inductor 4. This applies to theembodiments described below as well.

FIG. 3 shows a modification of the apparatus according to FIG. 1. Theinductor 16 is connected in parallel with a branch containing thehalf-wave rectifier 17 in series with the resistors 18 and 19, thesecond of which is connected in parallel with the back contact 20a ofthe delayed-action welding current relay 20. The resistor 18 correspondsto the resistor 11 of FIG. 1. The functioning of the apparatus duringthe start period is identical with the one described above. withreference to FIG. 1. In the present case, the rectifier branch remainsin circuit after the termination of the start period, but, owing to theinsertion of the resistor 19, with a reduced conductance in the forwarddirection of the rectifier l7.

In the apparatus according to FIG. 4, the inductor 21, as in theapparatus according to FIG. 1, is connected in parallel with a branchcontaining a rectifier 22 in series with a resistor 23 corresponding tothe resistor 11 of FIG. 1. The rectifier 22 is a silicon controlledrectifier (thyristor) the gate, or control electrode, 22a of which isconnected to a firing circuit extending from the positive terminal 25 ofthe source of power 24 through a resistor 27, a half-wave rectifier 28,a capacitor 29, the control electrode 22a, the resistor 23, the contacttube 30, the welding electrode 31, the workpiece 32 and the return cable33 to the negative terminal 26 of the source of welding power. Aresistor 34 corresponding to the resistor 12 of FIG. I is connected inparallel with the thyristor.

The thyristor is non-conductive when 'no welding current flows in thecircuit. When the welding process is initiated by the electrode tipbeing moved into contact with the workpiece, a current pulse inducingthe conductive state of the thyristor flows through the ignition circuitdescribed above. Said current pulse charges the capacitor 29 to avoltage determined not only by the constants of the ignition circuit butalso by the time interval between the moment in which the ignitioncurrent starts to flow and the moment in which the charg ing isterminated by the thyristors becoming conductive, said time intervalbeing determined by the properties of the thyristor. The functioningduring the short circuit period and the subsequent first ignition of thearc is identical with the one described above with reference to FIGS. 1and 2.

For the reason explained with reference to FIG. 1, it is advisable tohave the thyristor 22 in the conductive state during at least one shortcircuit period in addition to the first one. This necessitates re-firingof the thyristor, as it is restored to its non-conductive state at thetime t,. To this end, the time constant of the charging circuit isadjusted in such a way that at least two successive firing pulses arerequired to raise the potential of the capacitor 29 to such a level thatthe next charging impulse is too weak to cause firing of the thyristor22. i

The bleeder resistor (discharging resistor) 35 connected in parallel tothe capacitor 29 serves on termination of a welding operation todischarge the capacitor to permit re-firing of the thyristor. The timeconstant of the resistor-capacitor combination 29-35 may for instance be0.5 to 1 second and must under no circumstances be so small as to causethe capacitor to be discharged in the interval between two shortcircuits during the welding period to a level rendering the nextcharging pulse sufficiently strong to ignite the thyristor.

The following data are specified to exemplify the practical design of anapparatus'according to FIG. 4. The source of welding power is a weldingrectifier able to supply a welding current of 275 A at a duty cycle ofpercent. The inductance of the inductor 21 is 0.70

mil. The resistors 23 and 34 are of 60 and 220 milliohms, respectively.The limiting resistance 27 is 100 ohms, the capacitor 29 is lmicrofarad, and the discharge resistance 35 is l megohm. The apparatusis substantially intended for a semi-automatic welding with steelelectrodes having diameters between 0.6 and 1.2 mm. 1

In the apparatus shown in FIG. 5, a branch containing a thyristor 37 inseries with a resistor 38 is connected in parallel with the inductor 36,the thyristor being connected in parallel with two resistors 39, 40connected in series. The thyristor is provided with a firing circuit 41comprising the same elements and functioning in the same way as thefiring circuit of FIG. 4. This part of the apparatus works in a wayentirely identical with the one of the apparatus according to FIG. 4.The series connection of the resistors 38 and 40 is connected inparallel with a second thyristor 42 having a firing circuit 43. Thisthyristor is connected in opposition to the thyristor 37 and thus allowsin its conductive state a flow of current produced by the inductivevoltage occurring at decreasing currents in the inductor 36. The currenti, (FIG. 6) flowing through the electrode 44 during the arc period isnot equal to the current in the inductor winding, as in the embodimentspreviously described, a part i, of said current being bypassed by thebranch containing the thyristor 42. At the moment t, in which the arc isignited the electrode current is reduced suddenly from the short circuitvalue by an amount i +i,

Compared to the apparatus according to FIG. 4, the apparatus accordingto FIG. 5 has the advantage that at equal welding currents the load oneach of the two thyristors is lower than the load on the singlethyristor of FIG. 4.

In the above description the simplifying assumption has been made thatthe welding circuit contains no other inductance than the one of theinductor 4. In actual practice, additional inductance may be present.The source of welding power usually contains substantial inductance. Theshort circuit current therefore does not instantaneously rise to aninitial value 1' but grows from zero at a certain rate of rise orsteepness. An extra inductor may be inserted in the welding circuit tocontrol the initial rate of rise of the short circuit current.

Iclaim:

1. Apparatus for electric arc welding comprising a D.C. source ofwelding power, a fusible electrode, means for advancing said electrodetowards a workpiece at a constant rate, first circuit means connecting afirst terminal of said source of welding power to said electrode, secondcircuit means connecting a second terminal of said source of weldingpower to the workpiece,'said first circuit means having an inductorconnected in series between said first terminal and said electrode, ashunt branch connected across spacedapart points of a winding of saidinductor, said shunt branch containing a resistor in series with ahalf-wave rectifier poled to block a flow of current through said branchcaused by the voltage induced in said winding on decrease of the weldingcurrent through said inductor, and delayed operation switching meansconnected in said branch responsive with a delay not less than 0.3

second t o a flow of welding current for reducin the conductivity ofsaid branch in respect of currents aving the direction admitted by saidhalf-wave rectifier from an initial level to a substantially lowerlevel.

2. Apparatus for electric arc welding comprising a DC. source of weldingpower, a fusible electrode, means for advancing said electrode towards aworkpiece at a constant rate, first circuit means connecting a firstterminal of said source of welding power to said electrode, secondcircuit means connecting a second terminal of said source of weldingpower to the workpiece, said first circuit means having an inductorconnected in series between said first terminal and said electrode, ashunt branch connected across spacedapart points of a winding of saidinductor, said shunt branch having a resistor connected in series with asilicon controlled rectifier poled to block a flow of cur rent throughsaid branch caused by the voltage induced in said winding on decrease ofthe welding current through said inductor, a firing circuit connectedbetween the anode lead and the gate lead of the silicon controlledrectifier for causing a firing current to flow from the anode leadthrough said firing circuit into said gate as soon as a positive voltagedifferential appears between the. anode lead and the cathode lead ofsaid silicon controlled rectifier, and means for rendering said firingcircuit inoperative after a predetermined period.

3. Apparatus for electric arc welding comprising a DC. source of weldingpower, a fusible electrode, means for advancing said electrode towards aworkpiece at a constant rate, first circuit means connecting a firstterminal of said source of welding power to said electrode, a secondcircuit means connecting a second terminal of said source ofweldingpower to the workpiece, said first-circuit means comprising aninduction connected in series between said first terminal and saidelectrode, a shunt branch connected between spacedapart points of awinding of said inductor, said shunt branch containing a resistor inseries with a silicon controlled rectifier poled to block a flow ofcurrent through said branch caused by the voltage induced in saidwinding on decrease of the welding current flowing through saidinductor, and a firing circuit for said silicon controlled rectifiercomprising a capacitor, a charging resistor and a half-wave rectifierconnected in series between the anode lead and the gate lead of thesilicon controlled rectifier, said half-wave rectifier being poled so asto allow the flow of a positive current from said anode lead to saidgate lead and said capacitor having sufficient capacity to ensure thatat least two successive firing pulses are required to charge thecapacitor to a voltage at which subsequent charging pulses are too weakto fire the silicon controlled rectifier.

4. Apparatus as claimed in claim 3 in which the capacitor is shunted bya bleeder resistor providing discharge of the capacitor on conclusion ofa welding operation.

5. Apparatus as claimed in claim 3 which further comprises a bypassresistor connected between the anode lead and the cathode lead of thesilicon controlled rectifier.

1. Apparatus for electric arc welding comprising a D.C. source ofwelding power, a fusible electrode, means for advancing said electrodetowards a workpiece at a constant rate, first circuit means connecting afirst terminal of said source of welding power to said electrode, secondcircuit means connecting a second terminal of said source of weldingpower to the workpiece, said first circuit means having an inductorconnected in series between said first terminal and said electrode, ashunt branch connected across spaced-apart points of a winding of saidinductor, said shunt branch containing a resistor in series with ahalf-wave rectifier poled to block a flow of current through said branchcaused by the voltage induced in said winding on decrease of the weldingcurrent through said inductor, and delayed operation switching meansconnected in said branch responsive with a delay not less than 0.3second to a flow of welding current for reducing the conductivity ofsaid branch in respect of currents having the direction admitted by saidhalfwave rectifier from an initial level to a substantially lower level.2. Apparatus for electric arc welding comprising a D.C. source ofwelding power, a fusible electrode, means for advancing said electrodetowards a workpiece at a constant rate, first circuit means connecting afirst terminal of said source of welding power to said electrode, secondcircuit means connecting a second terminal of said source of weldingpower to the workpiece, said first circuit means having an inductorconnected in series between said first terminal and said electrode, ashunt branch connected across spaced-apart points of a winding of saidinductor, said shunt branch having a resistor connected in series with asilicon controlled rectifier poled to block a flow of current throughsaid branch caused by the voltage induced in said winding on decrease ofthe welding current through said inductor, a firing circuit connectedbetween the anode lead and the gate lead of the silicon controlledrectifier for causing a firing current to flow from the anode leadthrough said firing circuit into said gate as soon as a positive voltagedifferential appears between the anode lead and the cathode lead of saidsilicon controlled rectifier, and means for rendering said firingcircuit inoperative after a predetermined peRiod.
 3. Apparatus forelectric arc welding comprising a D.C. source of welding power, afusible electrode, means for advancing said electrode towards aworkpiece at a constant rate, first circuit means connecting a firstterminal of said source of welding power to said electrode, a secondcircuit means connecting a second terminal of said source of weldingpower to the workpiece, said first-circuit means comprising an inductionconnected in series between said first terminal and said electrode, ashunt branch connected between spaced-apart points of a winding of saidinductor, said shunt branch containing a resistor in series with asilicon controlled rectifier poled to block a flow of current throughsaid branch caused by the voltage induced in said winding on decrease ofthe welding current flowing through said inductor, and a firing circuitfor said silicon controlled rectifier comprising a capacitor, a chargingresistor and a half-wave rectifier connected in series between the anodelead and the gate lead of the silicon controlled rectifier, saidhalf-wave rectifier being poled so as to allow the flow of a positivecurrent from said anode lead to said gate lead and said capacitor havingsufficient capacity to ensure that at least two successive firing pulsesare required to charge the capacitor to a voltage at which subsequentcharging pulses are too weak to fire the silicon controlled rectifier.4. Apparatus as claimed in claim 3 in which the capacitor is shunted bya bleeder resistor providing discharge of the capacitor on conclusion ofa welding operation.
 5. Apparatus as claimed in claim 3 which furthercomprises a bypass resistor connected between the anode lead and thecathode lead of the silicon controlled rectifier.